Control of electrostatic precipitator current by electrical means



Feb. 18, 1958. H. KLEMPERER 2,823,757

CONTROL OF ELECTROSTATIC PRECIPITATOR CURRENT BY ELECTRICAL MEANS Filed. Feb. 11, 1954 2 Sheets-Sheet l "I g 4 I L unnum LIUIAJIE CONTROL 594 ll/GH-LOW SUPPL S INVENTOR. Hans A/em oere/" ATTORNEY STA T1 0 NARY PREC/D/ TA 70/? Feb. 18, 1958 H. KLEMPERER 2,823,757

CONTROL OF ELECTROSTATIC PRECIPITATOR CURRENT BY ELECTRICAL MEANS Filed Feb. 11, 1954 2 Sheets-Sheet. 2

e/Z aj INVENTOR Hans K/emperer NEY United States Patent CONTROL OF ELECTROSTATIC PRECIPITATOR CURRENT BY ELECTRICAL MEANS Hans Klemperer, Belmont, Mass., assiguor, by mesne assignments, to Apra Precipitator Corporation, New York, N. Y., a corporation of Delaware Application February 11, 1954, Serial No. 409,660

8 Claims. (Cl. 183-7) This invention is concerned with electrostatic precipitators for the removal from combustion or other impure gases of fine solid particles or impurities and relates particularly to improved means for automatically controlling the application of charging voltages to the precipitator electrodes during the particle collecting and cleaning or discharge cycles of such precipitators.

The precipitator in which the invention is embodied ettects removal of impurities without interruption from a continuously flowing column of any impure gases by electrostatic means incorporated in apparatus providing an ionizing section or zone followed by a collecting section or zone in which the particles previously electrostatically charged in the ionizing zone are deposited and collected on a suitable collecting surface. The precipitator is provided with collecting sections having a total cross sectional area for flow of gases greater than that required for flow of the gas column to be treated so that less than the total number of gas channels provided are utilized at a given time while the remainder of the channels are in a cleaning zone outside the path of flow of the gas column in order that the collecting surfaces may be cleaned without interrupting the gas cleaning function of the apparatus as a whole. In particular the invention contemplates apparatus of the kind described in which means are provided for automatically reducing the voltage applied to the charged elements of the precipitator prior to the cleaning period at which time the voltage may be completely out oli. The voltage is partially restored in a transition period and the charged elements have the full voltage again applied thereto as they fully resume their collecting function.

A salient feature of the invention is the provision of purely electrical control devices operative to automatically adjust the charging voltages applied to the precipitator electrodes to the conditions of operation of the precipitator. Specifically, a degenerative control circuit is employed to cause any variations in voltages at the electrodes due to operating conditions to be automatically reflected upon the related power supply so as to effect an appropriate adjustment of the charging voltage. The invention also contemplates employment of these electrical control means for extinguishing flashovers.

For a better understanding of the more detailed nature of the invention and the manner in which its several objects are attained, reference is made to the ensuing portion of this specification taken in conjunction with the accompanying drawings in which by way of example, but without limitation, an embodiment of apparatus for carrying the invention into efiect is disclosed. In the drawings:

Figure 1 is a schematic central longitudinal section through a gas cleaning apparatus embodying the invention;

Figure 2 is a diagrammatic view illustrating the electrical power supplies provided for applying an electrical tator; and

Figure 3 is a schematic wiring diagram showing the electrical control circuits for regulating the high voltage applied to the electrode elements of the precipitator.

Referring now more particularly to the precipitator shown in Figure 1, the reference character 10 indicates a duct delivering gases containing impurities from a furnace or other apparatus and 11 is a discharge duct for carrying away the cleaned gases. Between ducts 10 and 11 a stationary housing 12 is located. The housing 12 includes an inner cylindrical shell 13 spaced from the outer shell and joined thereto by a plurality of radially extending partitions to divide the annular space between the shells into a series of sector-like compartments providing the space for the collecting section 14 of the cleaner. The collecting surface is provided by a plate structure forming in each compartment a bank comprising a multiplicity of Open ended gas channels of hexagonal cross section.

Each of the gas channels is traversed longitudinally by a centrally located electrode 16. The electrodes 16 are supported by grids 15 of metal plate, there being one such grid for the bank of electrodes in each compartment of the housing. The grids are electrically insulated from the framework of the housing. At the lower or inlet ends of the gas channels other grids similar to grids 15 are provided, these also being supported by insulators. Current is supplied to each compartment through an individual feeder 18 so that through the structure just described the electrodes 16 may be electrically charged. In the lower portion of the space between the shells 12 and 13 the ionizing section 17 is located, the construction of which is similar to that previously described, the radial partitions that form the compartments being coextensive in length with the shells of the casing structure so that the ionizing section as well as the collecting section is divided and located in a series of compartments, the several parts of the ionizing section being also energized through the feed connections 18.

The casing structure is formed to provide a chamber 20 to which the gas inlet 10 leads and communicating through an annular opening 21 with the annular space in which the ionizing and collecting sections 17, 14 respectively are located. Within this chamber there is located a rotatably mounted hopper 22 projecting at its lower end through a suitable sealed opening 23 in the casing structure, being carried by a suitable bearing and having an external discharge outlet 24.

At the upper end of the shell structure a rotatably mounted casing 25 is provided, which comprises a sectorshaped wing 26 providing a chamber 27 housing a cleaning element in the form of a pipe 28 rotatable with the casing 25 by means of the suitable gearing. Pipe 28 is connected to a source of high pressure fluid, such as steam or air. The casing 25 operates to isolate a compartment to be cleaned from the compartments through which gas is flowing in the same manner as the hopper effects this separation at the lower end of the apparatus, and it willbe evident that as the hopper 22 and casing 25 are rotated electrode banks in different compartments can be successively cleaned without interruption to the flow through the apparatus of the gas to be cleaned.

The required cleaning may be intermittently or continuously effected without interruption of the continuous particle precipitating action applied to a continuously flowing column of gases. As will be apparent from the foregoing description of the apparatus, when the cleaning element 27 and hopper 22 are aligned in turn with the several compartments each is temporarily out off from the gas stream and bank of electrodes then located in the cleaning zone" may readily be cleaned of accumulated deposits by means of the jet blast which is arranged so that it can be applied to direct the blast through all 3 of the gas channels of the bank or compartment as it turns.

The foregoing precipitator structure is more fully described in the patent to Per Hilmer Karlsson, No. 2,5 82, 33 dated January 8, 1952, which also discloses forms in which the collecting sections and sometimes the ionizing sections rotate with respect to a stationary cleaning station;

In Figure 3 of the drawing the numeral 30 designates a three-wire alternating electrical current source to which the primary winding 31 of the power transformer 32 is connected. The secondary 33 of the transformer is connected to a rectifier 34 which converts the alternating current to direct current and through the wire 18 supplies a high voltage charging potential to the electrodes 16 of the electrostatic precipitator. Connected in series with the primary windings 11 of the power transformer 12 are three reactors 36, 37, 38, whose exciting windings 40, 41, 42, are energized from a separate source 39 of current through a rectifier 44. The primary windings 31 of power transformer 32 also have connected in parallel therewith three other reactors 45, 46, 47, whose energizing windings 50, 51, 52, are energized from a tertiary coil 55 coupled to the reactor primary of one of the series reactors (e. g. 36) through a rectifier 54.

During ordinary operation of a precipitator sector one may expect between twenty five and fifty fiashovers per minute. Flashovers are a common occurrence in precipitator operation and their appearance is an indication of proper voltage adjustment. Certain types of power supplies draw their power from a single phase line and deliver it to the precipitator in individual pulses. A fiashover usually disappears in the brief interval between two pulses. It is because of these intervals that flashovers ordinarily do not cause trouble for such a precipitator. Because of the relatively low gas velocity, such precipitators can afford to utilize power pulses, though there is no collecting action between pulses. The time each particle stays in the precipitator is long enough (about two seconds) to make up for the loss of collecting time. At the forty feet per second gas velocity contemplated with applicants apparatus, however, particles pass through our precipitator in less than one third second. Therefore, to obtain a high collecting efficiency, the powerless intervals have to be dispensed with and a continuous direct current applied such as derived from a three phase line and bridge type rectifier. The resultant difliculty isthe fact that the flashovers do not disappear but stay alive and render the affected sector inactive until an artificial currentless interval has been produced and the power reapplied thereafter. The instrumentality in each power supply that automatically takes care of this operation is called the flashover extinction or snufling circuit. In the form disclosed by applicant in his earlier application, Serial No. 352,829 filed May 1, 1953, it consists of a voltage sensitive electronic tube and a current sensitive mechanical relay. At a flas'hover frequency of twenty five per minute such relay would operate more than two million times per year and replacement of the relay may be expected probably every two months.

According to the present invention a power supply is provided which performs the just described operations but without electro-mechanical relays.

Automatic control of parallel reactors Reference is again made to Figure 3 which presents a simplified line diagram for one of the twelve power supplies connected to the twelve separate sectors of a precipitator as shown in Figure 2. As stated above the power circuit includes series reactors 36, 37, 38, and parallel reactors 45, 46, 47. It will be noted that the windings 50, 51, 52 for the parallel reactors are now automatically excited from the tertiary coil 55 of the series reactor 36. This excitation is supplied through a rectifier 54 and a time constant modifying resistor 56 to 4 the excitation coils 50, 51, 52, of the parallel reactors 45, 46, 47. As a result of this automatic excitation, the electro-mechanical relays previously considered necessary to operate the excitation circuit for the parallel reactors are eliminated. The excitation status of the parallel reactors 50, 51, 52 is always the opposite of that of the series reactors 36, 37, 38 and by proper choice of the circuit constants, it is given a snap action. When the unit is started up, the precipitator voltage stays low for a very short moment but then rapidly snaps up into the excited'status. The effect of this snapping action is to aid automatic clearance of flashovers as will be described later.

' The control reactor The excitation for the series reactors 36, 37, 38, is derived from an intermediate or control reactor 60 which serves to amplify the excitation power needed. The control reactor is so connected to the rectifiers 64 as to be of the self excited type displaying a high amplification and a very fast operating time. This self saturating control reactor consists of the combination of two separate reactor components 61, 62, and the rectifiers 64. Introduction of this control reactor 60 reduces the power level for control purposes from two hundred fifty watts, for example, to less than ten watts. At this low level the electro-mechanical control relays heretofore used can be dispensed with and all necessary switching for reducing or cutting off voltage during and restoring it after the cleaning period may be performed directly by the contacts 59A, 59B, 59C (Figure 2 also) of the rotary switch 59 associated with the cleaning apparatus of the precipitator as disclosed in application, Serial No. 224,356 filed May 3, 1951 (now Patent 2,672,947 dated March 23, 1954). The rotary switch 59 (Figure 2) operates with micro-switches which have an average life of five million operations at the applied low current level. As these contacts 59A, 59B, 59C (Figure 3) which reduce or cut off charging voltage during cleaning of the electrodes in each sector and then restore it to continue particle collection, operate not more than once per minute or about one half million times per year, one can expect five to'ten years of trouble free operation.

As shown in Figure 3, the ground resistor 65 between power rectifier 34 and ground is tapped and the voltage which corresponds to precipitator current is introduced into the control reactor excitation coils 66, 67, in a direction opposing the external excitation from the regulated power source 71. This is because one side of the reactor 60 magnetizing windings 61, 62. is connected to the direct current source 71 so as to have the same polarity with respect to ground as the low voltage side of the power rectifier 34. Thus, the feedback circuit 70 has a degenerative and a stabilizing elfect; the current in the precipitator sector limits itself by regulating the excitation of reactor 60 and hence the series reactors 3638. The position of the slider 68 on resistor 65 defines the amount of feedback which is to be allowed. The feedback is set so that a rise in current during fiashover is hardly noticeable. It is expected that due to the current limitation as derived from the feedback circuit 70 most of the fiashovers in the precipitator will be blown out by the high gas velocity. A further benefit derived from the current stabilizing. action of the feedback is a more stable performance of the precipitator under varying boiler load conditions.

The presence of the control reactor 60 considerably facilitates the extinction of flashovers. A fiashover is a secondary short circuit and manifests itself in a sudden breakdown of voltage (from 15,000 v. to about v.) and also by a simultaneous increase in current. Such rise in current in a large precipitator is dangerous for the ionizer striicture/ Flashovers behave differently according to their points of origin. If they originate from dusty spots, as on the electrodes, they are most persistent.

Since a precipitator sector does not remain clean during the length of its operating cycle, it is expected that some fiashovers may not be extinguished through the action of the feedback circuit 70 and may stay alive at the limited current level that results from functioning of the feedback circuit. Therefore, an additional extinction circuit is needed and the trigger impulse for the extinction circuit is taken from the voltage breakdown.

Flashover snujfing circuit The flashover snuffing circuit disclosed herein has no moving parts in contrast to all those previously utilized. An electronic switching device including two thyratron tubes 80, 81, short circuits the plate transformer 82 whenever a voltage breakdown occurs at any precipitator structure to which the circuit for these tubes is momentarily connected through the wire 84 connecting with the line 18. If transformer 82 is shorted because of a flashover, a connection is formed that allows current from power source 39 to rapidly deenergize the coils 61, 62 of the control reactor 60 through the demagnetizing rectifiers 83, in parallel at opposite polarity with rectifiers 64 of the control reactor 60. Thus, whenever a flashover occurs the control reactor 60 is almost instantly demagnetized, as current from line 39 flows through reactor coils 61, 62, in the sense opposite to normal excitation, d e-excitation of the series reactors 36, 37, 38 follows and the precipitator sector current disappears. Through snap action of the parallel reactors 45, 46, 47 the voltage for the precipitator section stays off for a definite small time interval which is sufiicient to clear the flashover. Upon extinction of the flashover the returning voltage for the related sector automatically blocks off the thyratrons 80, 81 and normal operation is restored. The voltage snaps back rapidly and service is restored. Total time for flashover clearance and restoration of service is of the order of one quarter secondfor a large precipitator.

In the event that flashovers occur more frequently in a selected predetermined interval of time than is considered desirable the voltage applied to the electrodes of the particular sector is automatically reduced after a predetermined number of flashovers occur. A separate tap of the ground resistor 65 is connected by wire 91 to a circuit including the condenser 92 so as to build up a charge in the latter as fiashovers are repeated. Upon the discharge of the condenser 92 the negative bias on the grid 93 of a gas filled electronic tube 94 is restored. The plate of tube 94 is connected through wire 96 to the positive side of the power supply 71. When tube 94 fires it energizes relay 97, the action of this relay opens contacts 98 introducing a resistance 99 to reduce the current supplied to magnetizing windings 66, 67 and causes the control reactor 60 to function to reduce the precipitator voltage as described above. The above described too frequent flashover circuit for counting fiashovers and correcting the condition is itself the subject of an application filed under Serial No. 445,092, filed July 22, 1954, in the name of Jeremy Welts (now Patent 2,771,150 dated November 20, 1946.

What I claim is:

1. In a power supply system for an electrostatic precipitator having an alternating current source, a high voltage transformer with its primary winding connected to said source, a power rectifier connected to the secondary winding of said transformer with its high voltage side connected to electrodes of said precipitator and its low voltage side connected to ground, a ground for said precipitator, saturable reactors connected in series with said primary winding, a control wind for said reactor series the improvement comprising; a control reactor of the self saturating type connected for energizing the control winding of said series reactor; a resistor connected between the low voltage side of said power rectifier and the precipitator ground so that voltage drop across the resistor is proportional to precipitator current; a direct current source of the same polarity with respect to ground as the low voltage side of said power rectifier; and excitation windings for said control reactor connected between a tap on said resistor and said direct current source and constituting a feedback connection so that the precipitator current flowing through said resistor opposes the constant control reactor excitation and thus renders the feedback connection through said control reactor degenerative.

2. In a power supply system for an electrostatic precipitator having an alternating current source, a high voltage transformer with its primary connected to said source, a power rectifier connected to the secondary winding of said transformer with its high voltage side connected to elec trodes of said precipitator, and its low voltage side connected to ground a ground for said precipitator; saturable reactor means connected in series with said primary winding and a control winding for said reactor, the improvements comprising; a control reactor of the self saturating type connected for energizing the control winding of said series reactor; means for energizing said control reactor; a demagnetizing rectifier connected in parallel with the rectifiers of said self saturating control reactor at such a polarity as to oppose the aforesaid excitation of the said control reactor; and means responsive to the precipitator voltage for energizing said de-energizing rectifiers so as to de-excite said control reactor in order to de-excite said series reactor and the precipitator.

3. In a power supply system for an electrostatic precipitator having an alternating current source, a high voltage transformer with its primary connected to said source, a ground for said precipitator, a power rectifier connected to the secondary winding of said transformer with its high voltage side connected to electrodes of said precipitator and its low voltage side connected to ground, saturable reactor means connected in series, with said primary winding a control winding for said reactor, the improvement comprising; a control reactor of the self saturating type connected for energizing the control windings of said series reactor; a resistor connected between the low voltage side of said power rectifier and the precipitator ground so that voltage drop across the resistor is proportional to precipitator current; a direct current source of the same polarity with respect to ground as the low voltage side of said power rectifier; magnetizing windings for said control reactor connector between a tap on said resistor and said direct current source and constituting a feedback so that the precipitator current flowing through said resistor opposes the control reactor excitation and thus renders the feedback connection through said control reactor degenerative; a demagnetizing rectifier connected across the main windings of said self saturating control reactor at such a polarity as to oppose the aforesaid excitation of said control reactor; and electronic switching means responsive to the precipitator voltage for energizing said de-energizing rectifier so as to de-excite said control reactor with the effect of de-exciting said series reactor and the precipitator.

4. In a power supply system for an electrostatic precipitator having an alternating current source, a high voltage transformer with its primary connected to said source, a ground for said precipitator, a power rectifier connected to the secondary winding of said transformer with its high voltage side connected to electrodes of said precipitator and its low voltage side connected to said ground, saturable reactor means connected in series and parallel, respectively, with said primary winding, control windings for said reactor the improvement comprising; a control reactor of the self saturating type connected for energizing the control windings of said series reactor; a resistor connected between the low voltage side of said power rectifier and the precipitator ground so that voltage drop across resistor is proportional to precipitator current; a direct current source of the same polarity with respect to ground as the low voltage side of said power rectifier; magnetizing windings for said control reactor connected between a tap on said resistor and said direct current sourceand constituting a feedback so that the precipitator current flowing through said resistor thus opposes the constant control reactor excitation and renders the feedback connection through said control reactor degenerative; and a tertiary coil coupled to the primary of said series reactor for energizing the control windings of said parallel reactor to operate the latter at an opposite state of excitation with respect to said series reactor.

5. In a power supply system for an electrostatic precipitator having an alternating current source, a high voltage transformer with its primary connected to said source, a ground for said precipitator, a power rectifier connected to the secondary winding of said transformer with its high voltage side connected to electrodes of said precipitator and its low voltage side connected to ground, and saturable reactor means connected in series and paralle'Lrespectively, with said primary winding, the improvement comprising; a control reactor of the self saturating type connected for energizing the control windings of said series reactor; means for energizing said control reactor; a demagnetizing rectifier connected in parallel with the rectifiers of said self saturating control reactor at such a polarity as to oppose the aforesaid excitation of the said control reactor; switching means responsive to the precipitator voltage for energizing said tie-energizing rectifiers so as to de-excite said control reactor with the effect of de-exciting said series reactor and the precipitator; and a tertiary coil coupled to the primary of said series reactor and a rectifier for energizing the control windings of said parallel reactors to operate the latter at an opposite state of excitation with respect to said series reactor.

6. In a power supply system for an electrostatic precipitator having an alternating current source, a high volt age transformer with its primary connected to said source, a ground for said precipitator, a power rectifier connected to the secondary winding of said transformer with its high voltage side connected to electrodes of said precipitator and with its low voltage side connected to ground, saturable reactor means connected in series and parallel, respectively, with said primary winding, control windings for said reactors, means for energizing the control wind ings to operate said parallel reactors at an opposite state of excitation with respect to said series reactors, the improvement comprising; a control reactor of the self saturating type connected for energizing the control windings of said series reactor; a resistor connected between the low voltage side of said power rectifier and the precipitator ground and constituting a feed back so that the voltage drop across the resistor is proportional to precipitator current; means for energizing said control reactor; a demagnetizing rectifier connected between the main windings of said self saturating control reactor at such a polarity as to oppose the aforesaid excitation of said control reactor; and switching means responsive to the precipitator voltage for energizing said de-energizing rectifiersso as to de-excite said control reactor in order to de excite said series reactor and the precipitators.

7. In a power supply system in an electrostatic precipitator having an alternating current source, a high voltage transformer with its primary connected to said source, a ground for said precipitator, a power rectifier connected to the secondary winding of said transformer with its high voltage side connected to electrodes of said precipitator and its low voltage side connected to ground, saturable reactor means connected in series and parallel, respectively, with said primary windings, control windings for said reactors; the improvement comprising a tertiary coil coupled to the primary of said series reactor for energizing the control winding of said parallel reactor to operate the latter at an opposite state of excitation with respect to said series reactor.

8. A power supply system for an electrostatic precipitator having an alternating current source, a high voltage transformer with its primary connected to said source, a power rectifier connected to the secondary winding of said transformer and to electrodes of said precipitator, a ground for said precipitator, saturable reactors connected in series and parallel, respectively, with said primary windings, and a coupling circuit for energizing the magnetizing windings to operate said parallel reactors at an opposite state of excitation with respect to said series reactors, a resistor connected between the low voltage side of said power rectifier and the precipitator ground so that the voltage drop across the resistor is proportional to the precipitator current, a direct current source of the same polarity with respect to ground as the low voltage side of said power rectifier, and magnetizing windings for said series reactor connected between a tap on said resistor and said direct current source so that the precipitator current flowing through said resistor opposes the constant series reactor excitation and renders the feedback connection through said series reactor degenerative.

References Cited in the file of this patent UNITED STATES PATENTS 2,139,232 Hysko Dec. 6, 1938 2,351,681 Haug June 20, 1944 2,582,133 Karlsson Jan. 8, 1952 2,672,947 Klemperer Mar. 23, 1954 2,297,740 Brown Oct. 6, 1942 

